Combine baking potato, parsnips, sweet potato and salt; let stand for 20 minutes.
Drain and squeeze excess moisture from potato mixture.
Transfer potato mixture to a large bowl.
Add onion, flour, and ground black pepper; toss well.
Stir in egg whites.
Heat 1 tablespoon oil in a large non-stick skillet over medium high heat.
Spoon 1/3 cup batter for each of 5 latkes into pan.
Cook 4 minutes on each side or until browned.
Repeat procedure with 1 tablespoon oil and remaining batter.